I have installed an ICS leak from my carrier (rom created by Motorola signed by my carrier but not released) and the calendar app has gone, I reinstalled the ROM from 2 different download sites and nothing, no one complains about this issue at all, some others just compalin some events don't appear but they have the app o.0

Has your calendar app really been wiped? (Try adding it as a shortcut from the app drawer.)

You can sometimes download an application from your handset manufacturer for your PC which provides a feature like "emergency firmware recovery". This should allow you to install the latest official release of Android.